Recently, there has been growing international awareness of global environmental protection, and accordingly attention has focused on hybrid vehicles (HEVs and PHEVs) and electric vehicles (EVs) that realize reductions in the usage of fossil fuels and CO2 emissions. Research and development on lithium-ion secondary batteries (LIB), which have a high energy density per unit volume and unit mass and are capable of being reduced in size, has been becoming more active in order to allow lithium-ion secondary batteries to be used as a power supply for driving a hybrid vehicle or an electric vehicle. At present, carbon materials are commonly used as negative electrode materials of lithium-ion secondary batteries. In addition to carbon, lithium salts of metals such as Si, Sn, Ti, and V, lithium salts of oxides of these metals, a carbon-metal hybrid material, and the like that have high energy density are still being studied.
Among carbon materials, graphite materials, generally having a high capacity, have been widely used for mobile electronic equipment and the like. Graphite materials, having a high energy density, are mainly used as negative electrode materials of vehicle-mounted batteries. On the other hand, attention is focused on non-graphitizable carbon materials, which have good input-output characteristics and cycle characteristics. In particular, it is necessary for batteries for hybrid vehicles to have good input-output characteristics used for starting a vehicle and obtaining regenerative energy and a long life property that allows a battery to be repeatedly charged and discharged for a long period of time. Thus, non-graphitizable carbon may be suitably used.
As an example of a non-graphitizable carbon material used as a negative electrode material of a lithium-ion secondary battery, a non-graphitizable carbon material produced from petroleum-derived pitch or coal-derived pitch has been reported (e.g., see Patent Literatures 1 to 4).